<!-- https://mvnrepository.com/artifact/org.javassist/javassist -->
<dependency>
    <groupId>org.javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.29.2-GA</version>
</dependency>
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ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("org.example.A");
byte[] bytes = cc.toBytecode();
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// 创建一个新的类，如果pool中已经有这个类，则会替换成新的
CtClass cc = pool.makeClass("org.example.A");

// 追加一个field
CtField cf = CtField.make("public String name;", cc);
cc.addField(cf);

// 追加一个方法
CtMethod cm = CtMethod.make("public String getName() { return this.name; }", cc);
cc.addMethod(cm);

// 追加一个构造函数，这里用了setBody，CtMethod也有该方法
CtConstructor ccs = new CtConstructor(null, cc);
ccs.setBody("{this.name =\"佚名\";}");
cc.addConstructor(ccs);

// 将字节码写入class文件
cc.writeFile();
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//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.example;

public class A {
    public String name = "佚名";

    public String getName() {
        return this.name;
    }

    public A() {
    }
}
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ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("org.example.A");

CtMethod cm = c.getDeclaredMethod("sayHi");

// setBody方法直接修改整个方法体内容
cm.setBody("{ System.out.println(\"HiHiHi\"); }");

// insertBefore在方法第一行插入代码块
cm.insertBefore("System.out.println(\"start\");");
// insertAfter在方法最后插入代码块，不会影响return的结果
cm.insertAfter("System.out.println(\"after\");");
// insertAt在方法指定的第几行插入
cm.insertAt(10, "System.out.println(\"line10\");");

// instrument是当前方法中，运行到某些时间点的时候的修改，例如MethodCall就是调用其他函数时候的修改
// NewExpr是创建异常的时候，最常用的是MethodCall其他的简单看一下即可。
cm.instrument(new ExprEditor() {
            public void edit(NewExpr e) throws CannotCompileException {}
            public void edit(NewArray a) throws CannotCompileException {}
            public void edit(MethodCall m) throws CannotCompileException {}
            public void edit(ConstructorCall c) throws CannotCompileException {}
            public void edit(FieldAccess f) throws CannotCompileException {}
            public void edit(Instanceof i) throws CannotCompileException {}
            public void edit(Cast c) throws CannotCompileException {}
            public void edit(Handler h) throws CannotCompileException {}
        });
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//修改第一个入参的值，然后运行
$1 = 0;
$_ = $proceed($$);
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Class c = cc.toClass();

Class c1 = cc.toClass(new Loader());
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<dependency>
    <groupId>net.bytebuddy</groupId>
    <artifactId>byte-buddy-agent</artifactId>
    <version>1.14.10</version>
</dependency>
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// 这是A类
public class A {
    public void sayHi() {
        System.out.println("hi");
    }
}

// 这是主类
public class Main {
    public static void main(String[] args) throws Exception {
        A a = new A();
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.get("org.example.A");
        cc.getDeclaredMethod("sayHi").setBody("{System.out.println(\"No hi\");}");
        byte[] bytes = cc.toBytecode();
        Instrumentation inst = ByteBuddyAgent.install();
        inst.addTransformer(new ClassFileTransformer() {
            @Override
            public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
                return bytes;
            }
        }, true);
        a.sayHi(); // 打印hi
        inst.retransformClasses(A.class);
        a.sayHi(); // retransform后打印No hi
    }
}
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// #root.sayHi是表达式内容，是说要运行根节点的sayHi方法
// 第二个参数就是根节点
A a = new A();
Object result = Ognl.getValue("#root.sayHi", a); // 打印hi，并返回null，因为没有返回值void
System.out.println(result); // null
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Point point = new Point(1, 2);
Object result = Ognl.getValue("#root.x=100, #root.y=200, #root.toString()", point);
System.out.println(result); // 打印java.awt.Point[x=100,y=200]
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Object result = Ognl.getValue("@java.util.UUID@randomUUID()", null);
System.out.println(result); //打印一个uuid
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